The present invention relates to components and testing methods for use in the production of electro-optic displays. This invention primarily relates to such components and methods containing an electro-optic medium which is a solid (such media may hereinafter for convenience be referred to as “solid electro-optic media”), in the sense that the electro-optic medium has solid external surfaces, although the medium may, and often does, have internal liquid- or gas-filled spaces, and to methods for assembling displays using such an electro-optic medium. Thus, the term “solid electro-optic displays” includes encapsulated electrophoretic displays, encapsulated liquid crystal displays, and other types of displays discussed below. The components and testing methods of the present invention are particularly, though not exclusively, intended for use in the production of electro-optic displays comprising electrophoretic media, especially encapsulated electrophoretic media.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870.657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Publication No. 2005/0001810; European Patent Applications 1,462,847; 1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067; 1,577,702; 1,577,703; and 1,598,694; and International Applications WO 2004/090626; WO 2004/079442; and WO 2004/001498. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; 6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603; 6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,430; 7,030,412; 7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502; 7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164; 7,116,318; 7,116,466; 7,119,759; and 7,119,772; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 2002/0180687; 2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0007336; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0099672; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122565; 2005/0134554; 2005/0146774; 2005/0151709; 2005/0152018; 2005/0152022; 2005/0156340; 2005/0168799; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0213191; 2005/0219184; 2005/0253777; 2005/0270261; 2005/0280626; 2006/0007527; 2006/0024437; 2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311; 2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619; 2006/0197736; 2006/0197737; 2006/0197738; 2006/0198014; 2006/0202949; and 2006/0209388; and International Applications Publication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 B1; and 1,145,072 B1.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and published U.S. Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in an article in the Sep. 25, 2003 issue of the Journal “Nature” and entitled “Video-Speed Electronic Paper Based on Electrowetting”. It is shown in U.S. Patent Publication No. 2005/0151709 that such electro-wetting displays can be made bistable.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Other types of solid electro-optic media, for example encapsulated or polymer-dispersed liquid crystals, can also be used in the present invention.
As discussed at length in the aforementioned U.S. Pat. No. 6,982,178 (see column 3, lines 63 to column 5, line 46), an electro-optic display normally comprises a layer of electro-optic material and at least two other layers disposed on opposed sides of the electro-optic material, one of these two layers being an electrode layer. The manufacture of such an electro-optic display normally involves at least one lamination operation. Although many of the components used in electro-optic displays, and the methods used for their manufacture, are derived from liquid crystal displays (LCD's), the LCD assembly process (which involves flowing liquid crystal between spaced substrates) cannot be transferred to solid electro-optic displays, since the solid electro-optic medium must be secured to layers on both sides and cannot simply be slid between the two surrounding layers. Accordingly, most prior art methods for final lamination of solid electrophoretic displays are essentially batch methods in which (typically) the electro-optic medium, a lamination adhesive and a backplane are brought together immediately prior to final assembly, and it is desirable to provide methods better adapted for mass production.
The aforementioned U.S. Pat. No. 6,982,178 describes a method of assembling a solid electro-optic display (including a particle-based electrophoretic display) which is well adapted for mass production. Essentially, this patent describes a so-called “front plane laminate” (“FPL”) which comprises, in order, a light-transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The term “light-transmissive” is used in this patent and herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will be normally be viewed through the electrically-conductive layer and adjacent substrate (if present); in cases where the electro-optic medium displays a change in reflectivity at non-visible wavelengths, the term “light-transmissive” should of course be interpreted to refer to transmission of the relevant non-visible wavelengths. The substrate will be typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to 254 μm). The electrically-conductive layer is conveniently a thin metal layer of, for example, aluminum or indium-tin-oxide (ITO), or may be a conductive polymer. Polyethylene terephthalate (PET) films coated with aluminum or ITO are available commercially, for example as “aluminized Mylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours & Company, Wilmington Del., and such commercial materials may be used with good results in the front plane laminate.
Assembly of an electro-optic display using such a front plane laminate may be effected by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium and electrically-conductive layer to the backplane. This process is well-adapted to mass production since the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for use with specific backplanes.
The aforementioned U.S. Pat. No. 6,982,178 also describes a method for testing the electro-optic medium in a front plane laminate prior to incorporation of the front plane laminate into a display. In this testing method, the release sheet is provided with an electrically conductive layer, and a voltage sufficient to change the optical state of the electro-optic medium is applied between this electrically conductive layer and the electrically conductive layer on the opposed side of the electro-optic medium. Observation of the electro-optic medium will then reveal any faults in the medium, thus avoiding laminating faulty electro-optic medium into a display, with the resultant cost of scrapping the entire display, not merely the faulty front plane laminate.
The aforementioned U.S. Pat. No. 6,982,178 also describes a second method for testing the electro-optic medium in a front plane laminate by placing an electrostatic charge on the release sheet, thus forming an image on the electro-optic medium. This image is then observed in the same way as before to detect any faults in the electro-optic medium.
Both of these testing methods have disadvantages. The first method requires an electrically conductive layer in the release sheet, and release sheets provided with such conductive layers are substantially more expensive than conventional release sheets which lack such conductive layers. Furthermore, this testing method requires that electrical contact be made with the conductive layer in the release sheet, and although this can readily be done when testing is effected on an isolated piece of front plane laminate (such as the piece clamped in an electrostatic chuck illustrated in the patent—one can simply peel the release sheet away from one corner of the piece to expose the conductive layer) it may be more difficult to do if is desired to test a front plane laminate in the form of a continuous web. In addition, the conductive layers on commercial release sheets incorporating such layers are normally opaque, so providing a release sheet with such an opaque conductive layer prevents visual inspection of one surface of the electro-optic layer in the front plane laminate during the testing.
The apparatus disclosed in the patent for effecting the second, electrostatic method, namely an ionographic print head, may also present problems. It may be difficult to provide sufficient electrostatic charge to ensure uniform switching of the electro-optic medium if the electro-optic medium is of a type (for example, an electrochromic medium) which requires substantial current flow for switching, or if the front plane laminate is in the form of a continuous web moving at substantial speed. It is normally desirable to test the electro-optic medium in both its extreme optical states to ensure detection of any defects which result in a particular area of the front plane laminate failing to switch, and testing both extreme optical states using the electrostatic method requires that a second electrostatic head remove the electrostatic charge applied by a first head and then apply an electrostatic charge of opposite polarity, which imposes further stress upon the ability of the second electrostatic head to apply sufficient charge to the front plane laminate. The presence of certain optional layers, especially anti-static layers, on the front plane laminate may interfere with the electrostatic method. Finally, an ionographic print head or similar electrostatic charge application device may be susceptible to edge effects which may render it difficult to ensure accurate testing of peripheral portions of a front plane laminate, especially the edges of a broad web of such material.
The aforementioned front plane laminate has proved to be very useful for assembly of electro-optic displays, especially flexible displays and displays with one rigid (typically glass) and one flexible substrate. However, a front plane laminate does have certain limitations. Since the FPL is normally produced by coating the electro-optic layer directly on to a substrate which will eventually form the front electrode (and typically an associated front protective layer) of the final display), the front electrode must be a coatable light-transmissive conductor, which restricts the choice of front conductor and protective layer. For example, some possible protective layers, such as glass or thick plastic layers, may be too thick and stiff to allow coating of the electro-optic medium, while other possible substrates may be too fragile for this purpose. Also, since practical mass production requires that the FPL be produced as a continuous web, waste occurs when portions of the web cannot be used because of the sizes and shapes of FPL pieces used to produce displays. As the complexity and cost of the substrate to be coated increases, the cost of the substrate can substantially affect the final cost the display. Moreover, a display production process using an FPL must make provision for establishing electrical contact between the front electrode within the FPL and the backplane, since in practice the backplane is provided with contacts to which are fed the voltages required at the front electrode As described in the aforementioned U.S. Pat. No. 6,982,178, it is necessary either to “clean” the electro-optic medium from one or more portions of the FPL to expose the front electrode layer and allow the necessary contacts to be formed, or to pre-form contact pads on the substrate before the electro-optic medium is coated thereon, the coating being effected so that electro-optic medium is not deposited on the contact pads, or any electro-optic medium deposited being removed later. While in general the use of pre-formed contact pads tends to be more satisfactory, it does have the disadvantage of reducing flexibility in the manufacturing process; since the location of the contact pads varies with the intended application of the FPL, pre-formed contact pads fix the application in which a particular roll of FPL can be used. In some cases where a barrier layer is incorporated into the substrate on which the FPL is formed, it may be necessary to clean a peripheral portion of the FPL to allow an edge seal to be formed. Finally, it is known that some electro-optic media are sensitive to moisture and to the relative humidity of the atmosphere. Accordingly, it is desirable to “condition” the electro-optic medium by allowing it to equilibrate with an atmosphere having standard temperature and relative humidity before the final display is sealed in order to ensure that the display has standardized electro-optic performance. It is difficult to condition an electro-optic medium derived within an FPL after the electro-optic medium has been placed between the front substrate and the lamination adhesive layer since the materials used for both the front substrate and the lamination adhesive are typically impervious to moisture, and this may lead to problems when final display assembly is conducted in poorly controlled environments.
The aforementioned 2004/0155857 describes a so-called “double release film” which is essentially a simplified version of the front plane laminate previously described. One form of the double release film comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release film comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate already described, but involving two separate laminations; typically, in a first lamination the double release film is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a backplane to form the final display. Although obviously the electro-optic medium can be tested after the first lamination (since the result of this first lamination is in effect a front plane laminate), it would be desirable to provide some method for testing the double release film prior to the first lamination.
The aforementioned copending application Ser. No. 11/550,114 describes a form of electro-optic display in which a lamination adhesive layer is present between the electro-optic layer and the front electrode through which an observer views the display; a second lamination adhesive layer may or may not be present between the electro-optic layer and the backplane of the display. This copending application also describes a so-called “inverted front plane laminate” which resembles the front plane laminate described in the aforementioned U.S. Pat. No. 6,982,178, but in which the positions of the electro-optic layer and the lamination adhesive layer are reversed, so that the inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer; an adhesive layer; a layer of a solid electro-optic medium; and a release sheet.
The aforementioned copending application Ser. No. 11/550,114 also describes an electro-optic display comprising a front member comprising at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer, the front member forming a viewing surface through which an observer views the display, an electro-optic layer; and an adhesive layer disposed between the electro-optic layer and the front member, the front layer having areas of at least two different colors so that the adhesive layer serves as a color filter.
The present invention seeks to provide improved testing methods for front plane laminates, double release films and other sub-assemblies used in the manufacture of electro-optic displays. The present invention also seeks to provide improved double release films, and similar sub-assemblies useful in the manufacture of electro-optic displays, adapted for use in the testing methods of the invention.